This invention relates to a pulse train converter for converting a first pulse train into a second pulse train of a higher frequency. More particularly, it relates to a pulse train converter which suitably serves to estimate the angle of rotation of a rotary shaft.
The pulse train converter according to this invention estimates effectively, in particular, the angle of rotation of the shaft of a gasoline engine. Thus an explanation will be made of the prior art device employed in combination with a gasoline engine.
The exhaust gas of gasoline engines is polluting the air and thus endangering our health and environment very alarmingly. The air pollution due to the exhaust gas of gasoline engines is now a serious social problem. Such air pollution is said to be checked appreciably if the gasoline engines are operated always under an optimum condition.
To operate each gasoline engine under an optimum condition the ignition should be so controlled as to take place at an appropriate time in accordance with the speed of rotation of the engine shaft. Various means are employed to determine the moment at which the ignition is to occur. Generally, a governor is used to measure the centrifugal force generated by the engine shaft rotation, or a diaphragm is used to measure the force by which the fuel is sucked into the engine. The force thus measured is utilized to shift the position where the ignition contacts are to meet. Thus, the ignition time is adjusted by shifting the ignition position. But the conventional ignition time point adjuster has a mechanically sliding section. The sliding section is liable to wearing, and is therefore likely to have the size of control part diminished. For this reason the conventional ignition time point adjuster cannot control accurately the ignition time point over a long time.
In an attempt to eliminate the drawback of the prior art device, thsre has been proposed such an ignition time point control device as shown in FIG. 1. This device has no contact. It comprises an optimum ignition angle determination (or decision) device 1, an engine ignition angle estimation device 2, a coincidence circuit 3 and an ignition circuit 4. An engine parameter is fed to the optimum ignition angle determination device 1 for determining the angle of rotation of the engine shaft at which the ignition is to be effected. Meanwhile, an engine reference pulse PR is fed to the engine ignition angle estimation device 2. The pulse PR is generated each time a specified point of the engine shaft passes by a specified fixed point.
Upon receipt of the engine parameter the optimum ignition angle determination device 1 produces a signal which denotes the optimum ignition angle. Similarly, upon receipt of the engine reference pulse PR the engine ignition angle estimation device 2 produces a signal which denotes the estimated engine ignition angle. These signals are supplied to the coincidence circuit 3. In response to these signals the coincidence circuit 3 may produce a coincidence signal, which is supplied to the ignition control circuit 4. Upon receipt of the coincidence signal the ignition control circuit 4 generates an output. The output of the circuit 4 is fed to an ignition plug (not shown).
In the optimum ignition angle determination device 1 there are stored the results of an engine test in the form of a numerical data table or numerical formular. Thus the device 1 determines the optimum ignition angle immediately, upon receipt of the engine parameter.
An engine ignition angle is estimated by the engine ignition angle estimation device 2 in the following manner. In a 4-cylinder engine, the upper dead point of each cylinder arrives each time the engine shaft rotates by 90.degree.. The arrival of the upper dead point is detected by a sensor (not shown). Then the sensor produces an output, in response to which an engine reference pulse PR is generated as shown in FIG. 2A. Pulses PR are thus generated one after another as the engine shaft rotates. Thus obtained is a train 5 of pulses PR, which shall be called a first pulse train hereinafter. The first pulse train 5 is converted into such a second pulse train 6 as illustrated in FIG. 2B. The second pulse train 6 consists of a number of pulses PP which divide each pulse period of the first pulse train into parts, the number of which is denoted by M. For example, each pulse period is divided into 90 parts. In this case, the angle of rotation of the engine shaft can be estimated to 10. This means that the angle of rotation of the engine shaft for a given time is estimated by counting the pulses PP. To simplify the explanation, hereinafter pulses PR will be called reference pulses or first train pulses, and pulses PP will be called dividing pulses or second train pulses. For the same reason, the engine ignition angle estimation device 2 will be called pulse train converter.
The conventional pulse train converter 2 has such a construction as shown in FIG. 3. Namely it comprises a pulse generator 11, a frequency divider 12, a counter 13, a register 14 and a preset counter 15. The pulse generator 11 produces pulses having a predetermined frequency f.sub.1. The pulses are supplied to the frequency divider 12. The frequency divider 12 divides the frequency f.sub.1 by M (i.e. divisor for dividing the pulse period of the first train pulses PR). Each pulse from the pulse generator 11 is thus divided into pulses having a frequency f.sub.2. The pulses of frequency f.sub.2 are supplied to the counter 13. Each time it receives a first train pulse PR (engine reference pulse), the counter is reset and start counting the pulses of frequency f.sub.2. Similarly, the register 14 stores the count N of the counter 13 each time it receives a first train pulse PR. The contents (i.e. count N) of the register 14 are preset in the preset counter 15. To the preset counter 15 the pulses of frequency f.sub.1 from the pulse generator 11 are supplied as subtraction pulses. That is, upon receipt of one pulse of frequency f.sub.1 the present counter 15 has its preset value reduced by one. Each time its preset value becomes zero, the preset counter 15 produces a second train pulse PP, which is also supplied to the register 14. In response to the second train pulse PP the register 14 supplies its contents (i.e. count N) to the preset counter 15. No matter how much the pulse period T.sub.PR of the first train pulse PR varies, second train pulses PP having a pulse period of T.sub.PR /M, the number of which is M, are produced during the pulse period of T.sub.PR.
However, the pulse train converter of FIG. 3 is faced with the following problems. While the shaft of an automobile engine is rotating at the highest speed possible, for example at 6000 r.p.m., the pulse period T.sub.PR of the first train pulses PR is about 2.5 ms. Let the pulse period of the second train pulses PP be denoted as T.sub.PP. Then, if the angle of rotation of the engine shaft can be measured to the degree of .DELTA.T, the following equation will be established: ##EQU1##
These equations teach that if T.sub.PR, M and T are to be 2.5 ms, 90 and 10.mu.S, respectively, f.sub.1 has to be 9MHz. The electronic circuits generally used for controlling the automobile engines are constituted chiefly by MOS transistors. The MOS transistors should therefore withstand a temperature variation from -40.degree. C to 125.degree. C and the operated by power source voltage T.sub.BAT of a 6 to 30V. Under such conditions the MOS transistors will fail to operate if their input frequency is as high as 9MHz.
To solve the above-mentioned problem, such a pulse train converter as shown in FIG. 4 has been proposed. This pulse train converter differs from that of FIG. 3 merely in that it is provided with a frequency divider 16 and a divider 17. The frequency divider 16 divides the output frequency f.sub.1 of the pulse generator 11 by 2.sup.n, where n denotes an integer. It produces pulses having a frequency of f.sub.3 (= f.sub.1 /2.sup.n). The output pulses of the frequency divider 16 are supplied to the preset counter 15 as subtraction signals. On the other hand, the divider 17 divides the count N stored in the register 14 also by 2.sup.n, thereby obtaining N/2.sup.n. The value of N/2.sup.n is preset in the preset counter 15.
The pulse train converter shown in FIG. 4 can operate basically in the same manner as the pulse train converter of FIG. 3, and yet the input frequency of the preset counter 15 can be lowered sufficiently. But the pulse period T.sub.PP * of second train pulses PP* from the preset counter 15 is shorter than the true pulse period T.sub.PP of second train pulses PP, as shown in FIGS. 5B and 5C. This results in an erroneous estimation of the angle of rotation of the engine shaft. To put it more precisely, generally the value of N/2.sup.n cannot be an integer; it usually consists of an integral portion and a decimal fraction. Its integral portion is preset in the preset counter 15, but its decimal fraction cannot be preset. If M = 256 and .DELTA.T = 25.mu.s, the pulse period T.sub.PP of second train pulses shown in FIG. 3 comes to have the true value of T.sub.PR /256 as shown in FIG. 5B. In this case, however, the pulse period T.sub.PP * of second train pulses PP* obtained by the pulse train converter of FIG. 4 comes to have a false value which is smaller than the true value and which is represented as follows: ##EQU2##
Consequently, if the pulse train converter of FIG. 4 is employed, the difference between the true and false values of the pulse period of second train pulses will be cumulated. An error will therefore be inevitable in estimating the angle of rotation of the engine shaft. For instance, if M = 255, the error E.sub.255 amounts to 2.sup.n .times. 25.mu.s in some cases. Apparently, the angle of rotation of the engine shaft cannot be estimated accurately.
Accordingly the object of this invention is to provide a pulse train converter which can convert a first pulse train into a second pulse train the pulses of which have substantially the same pulse period and divide the pulse period of the first train pulses into M parts (M denotes an integer), and in which the input frequency to the main part can be kept relatively low.